(1) Field of the Invention
The present invention relates to a test circuit and a test method, and particularly to a test circuit and a test method for testing amplifiers that convert photoelectric currents supplied from light-receiving elements into voltages.
(2) Description of the Related Art
A Photo Detector IC (PDIC) used for optical pick-up (hereinafter referred to as “optical pick-up PDIC”) is used for CD players, DVD players and the like in order to convert an optical signal from a CD or a DVD into an electric signal. The optical pick-up PDIC includes a light-receiving element which converts an optical signal into an electric signal, and an amplifier which converts a photoelectric current, supplied from the light-receiving element, into a voltage. A conventional test of the amplifier, included in the optical pick-up PDIC, (a test of, for example, electric characteristics and frequency characteristics) which is performed on occasions such as product shipment employs a method for measuring an output voltage of the amplifier that converts a photoelectric current into a voltage, by irradiating the optical pick-up PDIC with light. However, a test apparatus for irradiating the optical pick-up PDIC with light is costly. Also, there is a problem that such a test, in which the optical pick-up PDIC is irradiated with light, is time consuming, and thus results in a high PDIC cost.
In consideration of the above problem, the following method for testing an amplifier is known: a method in which an electric current is supplied to the amplifier from a test circuit built into an optical pick-up PDIC, rather than to a photoelectric current generated from a light-receiving element by being irradiated with light (refer to Patent Reference 1—Japanese Patent No. 3203996 for an example).
FIG. 1 is a diagram which shows a structure of a test circuit according to Patent Reference 1.
As shown in FIG. 1, a test circuit 500 according to Patent Reference 1 includes an electric current supplying unit 510, and n-1 amplifiers 522-52n. 
The electric current supplying unit 510 is a current mirror circuit which includes a plurality of bipolar transistors. By being applied with a voltage to a terminal 511 from outside the test circuit 500, the electric current supplying unit 510 supplies, to each of the amplifiers 522-52n, an electric current corresponding to the applied voltage. In addition, although not shown in the figure, a light-receiving element is connected to the input side of each of the amplifiers.
With the above described structure, the test circuit 500 according to Patent Reference 1 is capable of applying electric currents to the plurality of amplifiers by being applied with the voltage from outside. As a result, it is possible to measure characteristics of the amplifiers by measuring the voltages at terminals Vout, which are outputs of the amplifiers. Therefore, the test circuit 500 according to Patent Reference 1 enables a test of the plurality of amplifiers at a time, without having to irradiate light-receiving elements with light.